Polarization-dependent optical fibre amplifier

ABSTRACT

An optical fibre including a waveguide and at least one stress applying region is described. The waveguide is defined by a numerical aperture, and the stress applying region is defined by a depressed refractive index. The optical fibre is configured such that the waveguide supports at least two polarised fundamental modes, two polarized first second-order modes, and two polarised second second-order modes. The waveguide includes comprises a gain medium. The stress applying region, the waveguide and the disposition of the gain medium are such as to provide preferential guidance to at least one of the modes at an operating wavelength.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. National Stage filing of PatentCooperation Treaty (“PCT”) application serial number PCT/GB2003/004088,filed 24 Sep. 2003 (International Filing Date), which in turn claimspriority to United Kingdom (Great Britain) Patent Application SerialNumber GB0222252.9, filed 25 Sep. 2002.

FIELD OF INVENTION

This invention relates to apparatus comprising an optical fibre. Theapparatus may form the basis of high-power lasers and amplifiers.

BACKGROUND TO THE INVENTION

Many applications of optical fibres required control of eitherpolarisation or the mode shape. Particular examples are found inhigh-power fibre lasers and amplifiers whose optical output is furtherprocessed by polarisation-dependent devices such as modulators andfrequency converters. Although polarisation-maintaining fibres exist,these do not necessarily provide a single polarisation output,particularly at high powers where reliable polarising optics can beexpensive, unreliable, and difficult to procure. A requirement thereforeexists for an active fibre that can be incorporated into a high-powerfibre laser or amplifier and which enables the laser or amplifier to beoperable as a single-polarisation source.

Stimulated Brillouin scattering provides a limitation for high-powerfibre lasers and optical amplifiers. Light travelling down the fibreexcites an acoustic wave which reflects the light, the reflected lightbeing shifted in wavelength by the Brillouin wavelength shift. Differentglass materials have different Brillouin wavelength shifts and Brillouinbandwidths.

It is known that the stimulated Brillouin scattering threshold can beincreased by varying the materials along an optical fibre, by inducing atemperature gradient along a fibre, and by utilizing glasses havingdifferent Brillouin shifts across the cross section of a fibre.

It is also known that the stimulated Brillouin scattering threshold canbe increased using so-called large mode area fibres. Such fibres canhave relatively low numerical apertures and can be operated multi-modedand bent such as to provide higher losses for the higher-order modes ascompared to the fundamental mode.

There is a need for a fibre that can be used in single-frequency lasersand amplifiers. There is a related need for a fibre that has a higherstimulated Brillouin scattering threshold.

SUMMARY OF THE INVENTION

According to a non-limiting embodiment of the present invention there isprovided apparatus comprising an optical fibre having a waveguide and atleast one stress applying region: in which the waveguide is defined by anumerical aperture; the stress applying region has a depressedrefractive index; the optical fibre is configured such that thewaveguide supports at least two polarised fundamental modes, twopolarised first second-order modes, and two polarised secondsecond-order modes; the waveguide comprises a gain medium; and thestress applying region, the waveguide and the disposition of the gainmedium are such as to provide preferential guidance to at least one ofthe modes at an operating wavelength.

An advantage of the invention is that the apparatus can be readilyconfigured to provide a single polarised mode from a fibre laser, andcan do so at high power levels. This is particularly important when theoutput from the laser is to be processed by other devices such asfrequency converters or phase modulators.

The optical fibre may be bent.

The gain medium may comprise one or more rare-earth dopants.

The rare earth dopant may comprise one or more of Ytterbium, Erbium,Neodymium, Praseodymium, Thulium, Samarium, Holmium, Europium, Terbium,and Dysprosium.

At least one of the fundamental modes, the first second-order modes, andthe second second-order modes may be leaky at the operating wavelength.

The optical fibre is preferably configured to operate as asingle-polarisation optical fibre at the operating wavelength.

The optical fibre may be tapered along its length.

The waveguide may be tapered along its length.

The numerical aperture may correspond to an index difference less than0.0035.

The numerical aperture may correspond to an index difference less than0.003.

The numerical aperture may correspond to an index difference less than0.0025.

The numerical aperture may correspond to an index difference less than0.002.

The optical fibre may comprise a photosensitive region.

The photosensitive region may be at least partly in the stress applyingregion.

The photosensitive region may be at least partly in the waveguide.

The optical fibre may be defined by a stimulated Brillouin scatteringthreshold, and the fibre may have been exposed to ultraviolet radiationat least partly along its length in order to increase the stimulatedBrillouin scattering threshold.

The optical fibre may be defined by a stimulated Brillouin scatteringthreshold, and the optical fibre may have been exposed to heat treatmentat least partly along its length in order to increase the stimulatedBrillouin scattering threshold.

The apparatus may be in the form of an optical amplifying device. Theoptical amplifying device may provide single-polarisation operation. Theoptical amplifying device may be an optical amplifier, a laser, a masteroscillator power amplifier, or a source of amplified spontaneousemission. In use, the optical amplifying device may emit opticalradiation. The optical radiation may be pulsed, modulated or continuouswave.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described solely by way ofexample and with reference to the accompanying drawings in which:

FIG. 1 shows apparatus according to the present invention;

FIG. 2 shows an optical fibre comprising two stress applying regions;

FIG. 3 shows the fundamental modes of an optical fibre;

FIG. 4 shows the second-order modes of an optical fibre;

FIG. 5 shows transmission of an optical fibre with respect towavelength;

FIG. 6 shows the relative transmission of two polarisations in anoptical fibre;

FIG. 7 shows an operating window in which single-polarisation operationis achieved;

FIG. 8 shows the variation in extinction ratio with wavelength andazimuth;

FIG. 9 defines azimuth;

FIG. 10 shows the variation of wavelength with azimuth for a 15 dBextinction ratio;

FIG. 11 shows an optical fibre with a gain medium;

FIG. 12 shows a refractive index variation of an optical fibre;

FIG. 13 shows an optical fibre with photosensitive regions in the stressapplying regions;

FIG. 14 shows an optical fibre with photosensitive regions in the core;

FIG. 15 shows apparatus in the form of an amplifying optical device;

FIG. 16 shows an optical fibre comprising depressed index regions;

FIG. 17 shows the refractive index profile of the fibre shown in FIG.16;

FIG. 18 shows an optical fibre comprising longitudinally extendingholes;

FIG. 19 shows an optical fibre comprising a raised index ring around thecore;

FIGS. 20 to 23 show manufacturing steps to fabricate the fibre shown inFIG. 19; and

FIGS. 24 and 25 show optical fibres having ring cores.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

With reference to FIG. 1, there is provided apparatus comprising anoptical fibre 1 having a waveguide 2 and at least one stress applyingregion 3: in which the waveguide 2 is defined by a numerical aperture 6;the stress applying region 3 has a depressed refractive index; theoptical fibre 1 is configured such that the waveguide 2 supports atleast two polarised fundamental modes, two polarised first second-ordermodes, and two polarised second second-order modes; the waveguide 2comprises a gain medium 4; and the stress applying region 3, thewaveguide 2 and the disposition of the gain medium 4 are such as toprovide preferential guidance to at least one of the modes at anoperating wavelength. The modes are shown below in FIGS. 3 and 4 as modeas numbers 31, 32, 41, 42, 43 and 44.

By depressed refractive index, it is meant that the refractive index ofthe stress applying region 3 is less than the average refractive indexof the cladding 5.

The numerical aperture 6 is related to the angle emitted by light guidedby the waveguide 2 at one of its ends. This definition is more generalthan defining it with respect to the refractive indices of the core 7and cladding 5, and can, for example, be used with a greater range ofoptical fibres such as doped core and microstructured (or holey) fibres.

The waveguide 2 can be a microstructured fibre containing longitudinallyextending holes along its length. The holes can be filled with air orother material(s) with low refractive index.

The optical fibre 1 can be circular, oval, elliptical or have arectangular cross-section. The optical fibre 1 is preferably coated.

FIG. 2 shows a cross-section of an optical fibre 20 comprising asubstrate 22, stress applying regions 21, a core 23 and an innercladding region 24. The core 23 and inner cladding region 24 comprisethe waveguide 2. The stress applying regions 21 are typically doped withboron or boron co-doped with germania and typically have a depressedrefractive index with respect to the refractive index of the substrate22. The substrate 22 is typically silica. The gain medium 4 is shown asbeing in the inner cladding region 24, which is useful for reducing theoverlap (and hence increasing the potential amount of stored energy withthe fibre) between the guided modes and the gain medium 4.

FIG. 3 shows the x- and y-polarised fundamental modes 31, 32 of thefibre 20, labelled HE₁₁ ^(x) and HE₁₁ ^(y) respectively. The fundamentalmodes 31, 32 have a maximum amplitude located approximately at thecentre of the core 23.

FIG. 4 shows the four second-order modes 41, 42, 43, 44 of the fibre 20.These can be separated into x- and y-polarised first second-order modes41, 42 and x- and y-polarised second second-order modes 43, 44. Thesecond-order modes 41, 42, 43, 44 have a minimum located approximatelyat the centre of the core 23.

FIG. 5 shows the relative transmission 55 of the optical fibre 20measured versus wavelength 56 of the optical fibre 20 when bent comparedto when straight. Bending the optical fibre has the effect of increasinglosses of modes that are operating near to cut off. The cut-offs of thefour second modes 41, 42, 43, 44 are labelled as 51, 52, 53, 54respectively. The depressed index of the stress applying regions 21 hasthe effect of reducing the cut-off wavelength of the second second-ordermodes 43, 44. The depressed regions 21 provides preferential guidance tothe first second-order modes 41, 42 compared to the second second-ordermodes 43, 44.

FIG. 6 shows the relative transmission 61 of the optical fibre 20measured versus wavelength 56 for x-polarised light compared toy-polarised light.

FIG. 7 shows the relative transmission 76 of x- and y-polarised lengthversus wavelength 56 of a different length of the optical fibre 20,drawn to have a smaller diameter. The cut-off wavelengths 71, 72 of thetwo fundamental modes 31, 32, and the cut-off wavelengths 73, 74 of thetwo first second-order modes 41, 42 are identified.

FIG. 8 shows the extinction ratio 81 of y-polarised light compared tox-polarised light measured through the optical fibre 20 as a function ofwavelength 56 and azimuth θ 91 of the bend direction relative to thestress applying regions 21 as further defined in FIG. 9. The wavelengthvariation 101 for a 15 dB extinction is shown plotted against azimuth 91in FIG. 10. FIGS. 8, 9 and 10 demonstrate that the bend losses can betuned by bending the optical fibre 20 and by controlling the azimuth 91of the bend radius.

FIG. 11 shows an optical fibre 110 comprising two gain mediums 111. Thegain medium 111 can comprises one or more rare-earth dopants. The rareearth dopant comprises one or more of Ytterbium, Erbium, Neodymium,Praseodymium, Thulium, Samarium, Holmium, Europium, Terbium, andDysprosium. Preferably the rare earth dopant is Ytterbium or Erbium. TheErbium may be co-doped with Ytterbium. The disposition of the gainmedium 111 is that it provides preferential gain for the fundamentalmodes 31, 32 and the second second-order modes 43, 44 compared to thefirst second-order modes 41, 42. This is because the first second-ordermodes 41, 42 have a null along the axis. Thus when the optical fibre 110is bent, the depressed index regions 21 will cause leakiness of thesecond second-order modes 43, 44, and the bend will cause additionalleakage of the first second-order modes 41, 42. The preferential gainafforded by the disposition of the gain medium 111 will offset lossesseen by the fundamental modes 31, 32 induced by the bend.

Alternatively or additionally, the gain medium 4 can be located in thecore 23 in a region where the intensity of the fundamental modes 31, 32is greater than the intensity of the first second-order modes 41, 42.

FIG. 7 shows a wavelength window 79 in which the optical fibre 20operates as a single-polarisation optical fibre. A similar wavelengthwindow will exist for the optical fibre 110. It is preferred that theoptical fibre 110 is configured to operate as a single-polarisationoptical fibre at its operating wavelength. This is achieved byappropriate choices of the refractive index profiles, relativedimensions and the fibre diameter.

FIG. 12 shows the refractive index profile 120 across the x-axis of theoptical fibre 110. The refractive index of the core n_(co) 121 is raisedwith respect to the refractive index of the cladding n_(cl) 122. Therefractive index of the stress applying regions ns 123 is depressed withrespect to the cladding n_(cl) 122. The refractive index of the stressapplying regions ns 123 is typically depressed by about 0.0001 to 0.01with respect to the refractive index of the cladding n_(cl) 122.

The refractive index n_(co) 121 is typically raised by about 0.001 to0.005 with respect to the refractive index of the cladding n_(cl) 122.However, it may be advantageous to reduce the refractive index of thecore n_(co) 121 to 0.0005 to 0.001 with respect to the substrate 22 inorder to reduce the numerical aperture 6 of the fibre 110 further.

The refractive index difference between core and cladding is oftenexpressed as a numerical aperture defined by NA=(n_(co) ²−n_(cl)²)^(0.5).

In many applications, it is beneficial to increase the threshold atwhich non-linear effects such as stimulated Brillouin scattering andstimulated Raman scattering occur. This can be achieved by increasingthe spot size of the modes propagating in the fibre. One way to increasethe spot size is to decrease the numerical aperture 6 below that ofconventional telecom fibres, and to increase the diameter of thewaveguide 2. It is advantageous for the numerical aperture 6 to be lessthan 0.1, and preferably in the range 0.06 to 0.08. A numerical apertureof 0.1 corresponds to a refractive index difference between core andcladding n_(c0)−n_(cl) of around 0.0035 in a silica fibre. Numericalapertures of 0.06 to 0.08 correspond to refractive index differencesbetween core and cladding n_(co)−n_(cl) of around 0.0012 to 0.0022. Thenumerical aperture 6 can be decreased further to be in the range 0.02 to0.06, corresponding to index differences of around 0.0002 to 0.0012,which can be achieved using stress guidance and/or low core dopantconcentrations. Alternatively, these numerical apertures 6 can beachieved using silica cores and depressed claddings and/ormicrostructure holes in the cladding. By stress guidance, it is meantforming a waveguide by virtue of the photoelastic effect. In this case,the stress applying regions 3 stress fibre, and in particular the areasurrounding the core 7 which has the effect of raising the refractiveindex seen by x-polarised light. The core 7 can then guide x-polarisedlight without utilizing traditional core dopants such as germania orphosphorus.

The core 23 may comprise a more complex refractive index profile,including at least one ring and index depressions, the refractive indexn_(co) 121 thus being interpreted as an effective refractive index—thatis the equivalent refractive index of a conventional step-index fibrethat guides light with the same spot size as would be guided in theoptical fibre 110.

Further increases in the stimulated Brillouin threshold can be achievedby tapering the optical fibre 1 along its length. Additionally oralternatively, the waveguide 2 can be tapered along its length. Theoptical fibre 1 can be twisted along its length, either during the fibredrawing process or after the fibre is drawn. Changes in stress along itslength can also be achieved by bending or coiling the fibre withdifferent bend radii along its length.

FIG. 13 shows an optical fibre 130 comprising round stress applyingregions 131. Such an optical fibre is typically referred to as a PANDAoptical fibre. The design features of the optical fibre 130 arepreferably similar to those described with reference to FIGS. 2 to 12,except that the stress applying regions 131 are approximately circular.The stress applying regions 131 comprise a photosensitive region 132.The photosensitive region 132 may be boron doped silica co-doped withgermania.

FIG. 14 shows an optical fibre 140 in which the core 23 comprises aphotosensitive region 141. The photosensitive region may also be locatedwholly or partly in the core 23 and/or partly in the stress applyingregions 131. Alternatively or additionally, the photosensitive regionmay be located in the inner cladding 24 or substrate 22. Thephotosensitive region 141 may be germania doped silica, or if located inthe inner cladding 24 or substrate 22, germania silica co-doped withboron.

Exposing the optical fibre 130, 140 to ultraviolet light (for example asused in the manufacture of fibre Bragg gratings) or heat treating theoptical fibre 130, 140 will change the stresses within the fibrecross-section and can increase the stimulated Brillouin scatteringthreshold. It is preferred that such irradiation or heat treatment isvaried along the optical fibre 130, 140.

FIG. 15 shows apparatus in the form of an optical amplifying device 150comprising the optical fibre 153 and a source 151 of pump radiation 152.The optical amplifying device 150 may be an optical amplifier, a laser,a master oscillator power amplifier, or a source of amplifiedspontaneous emission. In use, the optical amplifying device 150 may emitoptical radiation 154. The optical radiation 154 may be pulsed,modulated or continuous wave.

It is preferred that in use the optical fibre 1 is bent. With referenceto FIGS. 8, 9 and 10, it is seen that depending on the design of theoptical fibre 1, single-polarisation operation at the operatingwavelength of the optical amplifying device 150 can be achieved byvarying the azimuth 91 of the optical fibre 1.

FIG. 16 shows a cross-section of an optical fibre 160 having the gainmedium 4 in the core 23. The optical fibre 160 also comprises adepressed cladding region 161 and two stress applying regions 162. Thepurpose of the depressed cladding region 161 is to cut-off the secondsecond-order modes 43, 44 defined in FIG. 4, and the purpose of thestress applying regions 162 is to induce stress birefringence and/or tocreate a window of single-polarisation operation 79 as defined in FIG.7. In certain embodiments of the present invention, the optical fibre160 may have one or both of the depressed cladding regions 161 andstress applying regions 162.

FIG. 17 shows the refractive index profile of the optical fibre 160along the x-axis defined in FIG. 6. The core 4 has a raised index 170,the depressed cladding regions 161 a depressed index 172, and the stressapplying regions 162 a depressed index 174 compared to the index 173 ofthe substrate 22. The refractive index 171 of the inner cladding 24 isapproximately matched to the substrate 22. The index 174 of the stressapplying regions 162 may alternatively be matched to the index 173 ofthe substrate 22, or may even be raised. It is preferred however todepress the index 174 of the stress applying regions 162.

FIG. 18 shows an optical fibre 180 comprising longitudinally extendingholes 181 instead of the depressed cladding regions 161 of FIG. 16. Theholes 181 can be filled with air or other low index material(s).

FIG. 19 shows an optical fibre 190 comprising two depressed-indexregions 191 and a raised index ring 192 surrounding the core 141. Such awaveguide design comprising the core 23 and raised index ring 192 is anexample of a so-called large-mode area design. Large mode area fibreshave fundamental-mode spot sizes significantly larger than found intraditional telecommunication fibres. The core 23 may have a refractiveindex difference compared to the substrate 22 of around 0.001 to 0.002and a diameter of around 15 μm to 25 μm. The ring 192 may have a raisedindex of around 0.0002 to 0.001 compared to the substrate 22 and anouter diameter of around 40 μm to 80 μm.

FIG. 20 shows the cross-section of an optical fibre preform 200comprising a depressed index ring 201. The preform is sawn along thelines 202 to provide four longitudinally extending sections 203.

FIG. 21 shows the cross-section of a optical fibre preform 210comprising a raised-index ring 211. The preform 210 is sawn along thelines 212 to provide four longitudinally extending sections 213.

FIG. 22 shows a rod-in-tube preform assembly 220 comprising a substrate222 comprising holes 223 configured to accept the sections 203 and 213and stress applying rods 221. The substrate 222 is preferably fusedsilica, and the holes 223 can be machined using ultrasonic drilling.

FIG. 23 shows the cross-section of the resulting fibre 230 drawn fromthe preform assembly 220. The fibre 230 has gain regions 4 and depressedindex regions 201 in the axis of the stress applying regions 191, andinner cladding 24 and raised index regions 211 in the perpendiculardirection. The method of manufacture described with reference to FIGS.20 to 23 is preferred to avoid preform shattering when highly-stressedregions are allowed to heat and cool.

The examples provided above concentrate have shown fibres havingtraditional step-index cores. Other designs of waveguides 2 are alsoencompassed by the invention, including microstructured (or holey)fibres. In addition, ring-doped cores are also beneficial in someapplications. FIG. 24 shows a ring doped fibre 240 in which thewaveguide 2 comprises an inner core 241 and an outer core 242. The innercore 241 has a refractive index 243 which is less than the refractiveindex 244 of the outer core 242. FIG. 25 shows a similar fibre 250 inwhich the refractive index 243 is depressed with respect to the index120. The gain medium 4 can be located in one or more of the inner core241, the outer core 242 and the cladding 5.

It is to be appreciated that the embodiments of the invention describedabove with reference to the accompanying drawings have been given by wayof example only and that modifications and additional components may beprovided to enhance performance.

The present invention extends to the above mentioned features takensingly or in any combination.

1. Apparatus comprising an optical fibre having a waveguide and at leastone stress applying region, wherein: the waveguide is defined by anumerical aperture; the stress applying region is defined by a depressedrefractive index; the optical fibre is configured such that thewaveguide supports at an operating wavelength at least two polarisedfundamental modes, two polarised first second-order modes, and twopolarised second second-order modes; the waveguide comprises a gainmedium; the optical fiber is disposed in a bend; and the stress applyingregion, the waveguide, the disposition of the gain medium, and the bendof the optical fiber are such: as to provide preferential guidance to atleast one of the modes at the operating wavelength; the two polarisedfirst second-order modes and the two polarised second second-order modesare leaky at the operating wavelength; and the optical fibre operates asa single polarisation optical fibre at the operating wavelength.
 2. Theapparatus of claim 1 wherein the gain medium comprises one or morerare-earth dopants.
 3. Apparatus according to claim 2 in which the rareearth dopant comprises one or more of Ytterbium, Erbium, Neodymium,Praseodymium, Thulium, Samarium, Holmium, Europium, Terbium, andDysprosium.
 4. The apparatus of claim 1 wherein the optical fibre isdefined by a length, and is tapered along its length.
 5. The apparatusof claim 1 wherein the waveguide is defined by a length, and is taperedalong its length.
 6. The apparatus of claim 1 wherein the numericalaperture corresponds to an index difference less than 0.0035.
 7. Theapparatus of claim 1 wherein the numerical aperture corresponds to anindex difference less than 0.003.
 8. The apparatus of claim 1 whereinthe numerical aperture corresponds to an index difference less than0.0025.
 9. The apparatus of claim 1 wherein the numerical aperturecorresponds to an index difference less than 0.002.
 10. The apparatus ofclaim 1 wherein the optical fibre comprises a photosensitive region. 11.The apparatus of claim 10 wherein which the photosensitive region is atleast partly in the stress applying region.
 12. Apparatus according toclaim 10 in which the photosensitive region is at least partly in thewaveguide.
 13. The apparatus of claim 1 wherein the optical fibre isdefined by a stimulated Brillouin scattering threshold, and the opticalfibre has been exposed to ultraviolet radiation at least partly alongits length in order to increase the stimulated Brillouin scatteringthreshold.
 14. The apparatus of claim 1 wherein the optical fibre isdefined by a stimulated Brillouin scattering threshold, and the opticalfibre has been exposed to heat treatment at least partly along itslength in order to increase the stimulated Brillouin scatteringthreshold.
 15. The apparatus of claim 1 wherein the apparatus is in theform of an optical amplifying device.
 16. The apparatus of claim 15wherein the optical amplifying device is configured to providesingle-polarisation operation.
 17. The apparatus of claim 15 wherein theoptical amplifying device is an optical amplifier, a laser, a masteroscillator power amplifier, or a source of amplified spontaneousemission.
 18. The apparatus of claim 1 wherein the optical fibrecontains longitudinally extending holes along its length.
 19. Theapparatus of claim 1 wherein the bend is defined by an azimuth, and bendlosses caused by the bend are tuned by controlling the azimuth.
 20. Theapparatus of claim 1 wherein the optical fibre comprises a depressedcladding region.